Dual agency is not unethical, but may not necessarily be legal in some states. Some agents choose not to do dual agency even if it's allowed in their states.
Here's a link to dual agency in Pennsylvania http://membrane.com/philanet/benigno/pa_law.html
And here's how one agent feels about dual agency http://activerain.com/blogsview/12648/why-i-don-t-practice-dual-agency
In the end....think of it this way; when one is in court, isn't it normally the case for each person to have his own representation? By the same token, in court, it is generally an adversarial position, and someone pays
On the other hand, if both parties come to a mutually satisfying agreement using only one person to present both cases, and when both parties approach the process intelligently and logically, that could be an advantage/benefit as well.